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CIGRE 2016
Primary control reserves provision with battery energy storage systems in the
largest European ancillary services cooperation
Michael KOLLER1*, Marina GONZÁLEZ VAYÁ1, Aby CHACKO2,
Theodor BORSCHE3, Andreas ULBIG3
1
Elektrizitätswerke des Kantons Zürich (EKZ)
2
Swissgrid AG
3
ETH Zürich
Switzerland
SUMMARY
The Transmission System Operators (TSOs) of Germany, Austria, the Netherlands and Switzerland
started a joint ancillary services market for Primary Control Reserves (PCR) in April 2015. The
participating TSOs are currently discussing the harmonisation of prequalification rules for units with
limited storage capacity. The exact terms of these prequalification rules will eventually determine how
cost-effective the provision of ancillary services by energy-constrained units is.
The paper presents the technical, operational and regulatory suitability of Battery Energy Storage
Systems (BESSs) to bid into this largest European ancillary service cooperation, and gives results
from the operation of the Zurich 1 MW BESS in this market.
The main challenge for batteries when providing control reserves is the State of Charge (SoC)
management: the frequency error signal might be biased over prolonged periods of time, leading to the
battery running either full or empty. Since the required investment for BESS largely depends on the
storage capacity, the development of the new ENTSO-E Operational Guidelines and the
prequalification requirements have a profound impact on the business case of BESS for PCR
provision. The recharging strategy used by the Zurich 1 MW BESS is presented and compared to other
recharging methods. Possible new capacity requirements for batteries arising from the new
Operational Guidelines and new prequalification rules are evaluated by means of simulations and
compared to the measurements and operational experience from the Zurich 1 MW BESS. Simulations
suggest that a battery capacity of around 220 kWh per offered MW of PCR would be sufficient to
withstand even a worst-case frequency excursion, a fact which is supported by the accumulated
operational experience and SoC measurements during more than one year of operation. However,
depending on the development of the new Operational Guidelines and the recharging strategies
allowed under the framework of the new prequalification requirements up to 1’220 kWh of capacity
per MW of PCR could be necessary. Such an up to six-fold increase in required energy storage
capacity would severely put into question the economic viability of BESS projects for providing PCR
services to grid operators.
KEYWORDS
Battery Energy Storage Systems – Frequency Control Reserves – Ancillary Services Markets
[email protected], [email protected], [email protected],
[email protected], [email protected]
Introduction
In recent years there has been a growing interest in Battery Energy Storage Systems (BESS) for
providing necessary Ancillary Services such as frequency control to Transmission System Operators
(TSOs). Due to advances in battery technologies and associated significant cost reductions as well as
increased cycle life-times, notably for Li-Ion based battery systems, global installed grid-connected
BESS capacity has increased almost six-fold in the last 10 years, from 120 MW to 690 MW, according
to the International Energy Agency (IEA) [1].
This represents in fact a renewed interest in batteries for grid applications, as there had been a few
large-scale lead-based BESS projects as early as the 1980s in Europe [2] and the United States [3],
which, however, were eventually discontinued due to declining economic profitability.
Due to the ongoing power market liberalization, Ancillary Services such as frequency control are
nowadays often procured via auction-based mechanisms, making them an attractive market field for
battery unit operators.
In Europe, Ancillary Services are increasingly procured on an international level. The TSOs of
Germany, Austria, the Netherlands and Switzerland, for instance, started a joint ancillary services
market for Primary Control Reserves (PCR) in April 2015. The participating TSOs are currently
discussing the harmonisation of prequalification rules for units with limited storage capacity, i.e.
batteries. It is clear that the technical details of the finalized prequalification rules will have a strong
impact on the business case of battery system operators.
In this paper, we discuss the technical, operational and regulatory suitability of BESSs to bid into this
largest European Ancillary Services cooperation, and give results from the operation of the Zurich
1 MW BESS in this Ancillary Services market. We focus specifically on the impacts that a more
conservative set of prequalification rules would have on the effective integration of BESS units for
primary frequency control provision.
Part I: Technical suitability of BESS units
The main task of PCR is to rapidly stabilize grid frequency deviations after a fault event, before
secondary control eventually relieves primary control and brings system frequency back to its nominal
value. Currently, units providing PCR have to respond to a full activation within 30 seconds. This rate
of activation prevents extreme frequency deviations even in the face of a large contingency, such as
the disconnection of a large nuclear power plant. However, as more and more Renewable Energy
Sources (RES) such as wind turbines and Photovoltaics (PV) are integrated into the grid, the average
rotational inertia of the Continental European (CE) interconnection decreases. Therefore, grid
frequency dynamics become more erratic, rendering the traditional control reserve scheme
increasingly insufficient.
Figure 1: Simulated frequency response of the Continental European interconnection to a major power
fault for different levels of inertia H and time constant of primary response t. Load: 330 GW, fault: 3 GW.
Figure 1 shows a simulation of the frequency evolution after a 3 GW loss of generation contingency in
the CE grid, which is the worst-case contingency assumed by the European Network of TSOs for
2
Electricity (ENTSO-E). The blue line shows the frequency evolution with the current inertia and
reserve framework. The red line is with an inertia reduced to one third, as it could reasonably occur
during hours of low load and very high infeed from inverter-coupled RES. Finally, the green line
shows the frequency evolution with the same reduced inertia as with the red line, but also faster
PCR. The simulations highlight the effect of reduced inertia on the dynamic response of a power
system, and the benefit of fast-responding control reserves: With the reduction of inertia, the worstcase frequency deviation increases from 200mHz (blue; nominal system parameters) to 270mHz (red;
low inertia). Fast-responding PCR can mitigate the effects of low inertia and are effective in limiting
unwanted frequency excursions (green; low inertia and fast PCR). Such fast response can be provided
by BESSs.
System operators have identified decreasing rotational inertia as a main challenge, especially in small
interconnections with high shares of wind production such as Ireland and Texas. The corresponding
TSOs, EirGrid and ERCOT, respectively, propose to either provide virtual synchronous inertia or to
promote faster frequency response with significantly higher power ramp rates than currently
required [4, 5].
BESS based on Li-Ion cells are from a technological perspective well suited to provide fast PCR as
they can react rapidly to frequency deviations, i.e. at least an order of magnitude faster than thermal or
hydro power plants. Also, in normal PCR operation, only very little energy needs to be exchanged
with the grid. Even during contingencies, the energy requested from PCR is rather small. An
additional benefit of BESS PCR is the decoupling of energy production from reserve power provision.
This has both economic and operational advantages. Economic, as BESS units do not have an
opportunity cost for reserve provision. Operational, as must-run generation for reserve provision [6],
that otherwise might displace renewable generation, can be reduced. Reserve power provision by
BESS units is thus less dependent on the volatile electricity prices of power markets.
The main challenge for batteries when providing control reserves is the State of Charge (SoC)
management: the frequency error signal might be biased over prolonged periods of time, leading to the
battery running either full or empty. Moreover, the BESS efficiency losses need to be compensated.
Several battery recharging strategies are known: scheduled recharging [2], recharging while in the
dead-band around nominal grid frequency [7], moving average recharging [8] or recharging depending
on predefined SoC limits [9].
Part II: State of Charge Control with the Zurich 1 MW BESS
The Zurich 1 MW BESS depicted in Figure 2 was commissioned in March 2012. It has a rated power
of 1.1 MW and an energy capacity of 580 kWh, of which 250 kWh are usable at peak power in order
to limit battery degradation (see Table 1 for key properties) [10].
Figure 2: Top view of the Zurich 1 MW BESS.
3
Table 1: Key technical properties of the Zurich 1 MW BESS.
Property
Value
Comment
1 MW
1.1 MW
1.3 MW
580 kWh
ABB
Li-Ion
80 – 90 %
PCR power
- Rated power
- Peak power capability
Capacity
System Integrator
Technology
Efficiency
charging and discharging
250 kWh @ 1 MW
cells from LG Chem
round trip incl. auxiliary
The battery system prequalified for the provision of PCR in July 2014 according to the requirements
of the Swiss TSO. Since then, the battery unit participated continuously in the Swiss market as well as
in the newly formed joint PCR market, winning tenders for 41 weekly bid periods as of 31.12.2015.
The employed approach for controlling the SoC of the Zurich 1 MW BESS is a moving average
strategy [8] according to:
𝑘
1
𝑃off (𝑘) =
∑ −𝑃PCR (𝑖) + 𝑃loss (𝑖)
𝑎
𝑖=𝑘−𝑎
where 𝑃off is the recharging power to stabilize the SoC, 𝑎 is the averaging period, 𝑃PCR is the power
provided by the BESS according to a linear function in response to the local frequency measurement,
as required from all PCR units, and 𝑃loss is the sum of all power losses in the battery system. It allows
for very smooth recharging profiles, and succeeds in keeping the SoC close to nominal levels. The
recharging power is then simply added as an offset to the nominally requested PCR power:
𝑃BESS,tot (𝑘) = 𝑃PCR (𝑘) + 𝑃off (𝑘)
For the Zurich 1 MW BESS an averaging period 𝑎 of 15 minutes (180 steps, with a step duration of
5 s) was chosen to ensure a slow recharging behavior, and a simple power-dependent efficiency model
was used to calculate the losses. According to the peak power capability of 1.3 MW, 𝑃off is limited to
±0.3 MW to make sure the BESS can provide 1 MW of PCR power at all times.
Figure 3: Measurements from PCR provision during the outage of a nuclear power plant (1.275 GW)
at 4:46 am UCT (21/01/2015). The BESS used the moving average algorithm to stabilize the SoC.
4
The needed recharging energy is sourced from secondary frequency control. It was verified in [11] by
means of simulations that this control strategy performs well during contingencies, does not put
secondary control reserves under undue stress, and increases energy demand from secondary reserves
by only 1–2% over the course of one full year of operation. Under the real life conditions of the
Zurich 1 MW BESS, 𝑃off contained on average 0.4 MWh charging and 0.38 MWh discharging energy
per day, which was sourced from balancing energy including secondary control reserves (SCR).
During the operation of the Zurich 1 MW BESS in the PCR market, the minimum SoC was 44.5% and
the maximum SoC was 75.7%. The highest SoC level of 75.7% was caused by a long-lasting large
unidirectional frequency deviation of an average of 80 mHz during the hour of the switch from
summer time to winter time on 26 October 2014. Apart from this event, the maximum SoC during
PCR provision was 61.8%. The distribution of measured SoC values during PCR operation depicted in
Figure 4 clearly shows the capability of the moving average algorithm to stabilize the SoC at a
medium level slightly below 55%, while providing regular PCR services, even though the storage
capacity of the Zurich 1 MW BESS is with 580 kWh small in comparison with possible new capacity
requirements for PCR batteries (see also Table 4 in Part IV).
Figure 4: SoC histogram of the Zurich 1 MW BESS during PCR operation (nominal SoC level ≈ 55%).
Part III: Market structure for primary control reserve provision
The amount of PCR which has to be provided by each member country of the European Network of
TSOs for Electricity ENTSO-E is decided by ENTSO-E depending on the power generation of each
country on a yearly basis. In the ENTSO-E CE synchronous area, a total of 3000 MW are procured.
This corresponds to its designed worst-case fault, i.e. the simultaneous loss of two large nuclear power
plants.
Swissgrid started to source PCR by means of auctions in 2009. Beforehand, the major vertically
integrated Swiss utilities were responsible for providing PCR for Switzerland. After the introduction
of PCR auctions, Swissgrid has continuously adapted the market rules in order to increase the liquidity
in the PCR market, as well as to ensure the most economical procurement of PCR reserves. The major
changes that have been implemented after the introduction of PCR auctions included the change to
Pay-as-Bid from the Marginal Pricing method, the reduction of the minimum bidding amount from ±3
MW to ±1 MW and the change from monthly to weekly delivery periods, which increases flexibility
on the side of the PCR providers.
The newly introduced joint PCR market is based on the TSO–TSO model. Austria, Germany, the
Netherlands and Switzerland are participants, making it the largest common market for PCR in Europe
(see Table 2).
5
Table 2: PCR requirements for each country in the joint PCR market.
Country
PCR Amount for 2016
65 MW
Austria
583 MW
Germany
102 MW (71 MW is procured in the joint auction)
Netherlands
74 MW
Switzerland
793 MW
Total PCR amount in the joint auction
In comparison to the TSO–BRP (Balance Responsible Party) model, the individual product definitions
of each TSO can be retained inside the TSO–TSO model. These individual characteristics are taken
into consideration by the central market clearing algorithm. The other important advantage of this
model is that the market participants only have legal ties with their local TSO and can continue to use
the established bidding platforms of their TSO only.
Figure 5: TSO-TSO Model.
The principal idea of the TSO–TSO cooperation model is illustrated in Figure 5. The bidders give their
bids into the bidding system of their TSO. The gate closing is simultaneous and each TSO system
transmits its bids to the Central Clearing System (CCS). The CCS calculates the results and reports
these to the individual TSO systems. The TSO systems verify and accept the results. The results are
then published on the TSO systems and the bidders can view the results in their TSO system. New
participating TSOs can be seamlessly integrated within the TSO–TSO model.
The individual bidders will have to interact with only one system and hence have only one end-to-end
process. The CCS for the joint PCR co-operation was developed and is operated by Swissgrid, the
Swiss TSO.
One of the aims of a common PCR market is that it is opened up to participants from different
countries. This should lead to a price convergence. Figure 6 shows the development of average PCR
auction prices in Germany, Switzerland and Austria from 2013 to 2016. The prices have been
converted from Swiss Francs to Euro where applicable. The visible converging prices of accepted
PCR bids from the participating countries indicates that the correct market signals are being generated.
6
Austria
PRL_13_CW02
PRL_13_CW08
PRL_13_CW14
PRL_13_CW20
PRL_13_CW26
PRL_13_CW32
PRL_13_CW38
PRL_13_CW44
PRL_13_CW50
PRL_14_CW04
PRL_14_CW10
PRL_14_CW16
PRL_14_CW22
PRL_14_CW28
PRL_14_CW34
PRL_14_CW40
PRL_14_CW46
PRL_14_CW52
PRL_15_CW06
PRL_15_CW12
PRL_15_CW18_INT
PRL_15_CW24_INT
PRL_15_CW30_INT
PRL_15_CW36_INT
PRL_15_CW42_INT
PRL_15_CW48_INT
PRL_16_CW01_INT
Price [Euro/MW/h]
180
160
140
120
100
80
60
40
20
0
Switzerland
Germany
Calendar Weeks
Figure 6: Development of average PCR auction prices in Germany, Austria and Switzerland since 2013.
The convergence of PCR prices after the start of the joint auctions in April 2015 is clearly visible.
Outlook of joint PCR market
ENTSO-E has the aim of developing market-based cooperation and implementing the steps for
regional and European integration. This in turn will lead to increased economic efficiency, while also
ensuring operational security. ENTSO-E has proposed several cross-border pilot projects with the
purpose of testing the feasibility of the European (target) model and intermediate steps, evaluate the
associated implementation impact, and to report on the experience gained. The common PCR auction
has been selected as the pilot project for PCR cooperation [12].
Neighboring TSOs have expressed their interest in joining the newly formed PCR market for the
procurement of their respective PCR requirements. Since this model has been proven to work
successfully and the benefits of the joint procurement are beyond doubt, this cooperation has the
potential to become a pan-European cooperation for PCR procurement. It can also serve as a model for
the joint procurement of other control reserves.
Part IV: Evaluation of new prequalification rules
The technical requirements of ENTSO-E for the control reserves are described in the Policy 1 LoadFrequency Control and Performance [13]. The ENTSO-E is working on new Operational Guidelines
which contain the three operational codes Operational Security, Operational Planning & Scheduling,
and Load Frequency Control & Reserves. The Operational Guidelines have entered the comitology
process in December 2015. Hence, it is expected that they will enter into force in 2017, after approval
by the European Commission. The code for Load Frequency Control and Reserves will then replace
ENTSO-E’s Policy 1. In view of the new requirements of the Operational Guidelines, the TSOs
involved in the cooperation for the joint procurement of PCR are working together to define
harmonized rules for the prequalification of PCR delivery units and specifically BESS units. These
new harmonized rules are expected to be finalized by mid-2016. Some TSOs, such as the German and
the Austrian TSOs, have already defined a set of rules specific to energy-constrained PCR units, see
[14] and [15], respectively.
According to the draft version of the Operational Guidelines which follow the concepts outlined in
[15, 14], PCR providing units/groups are subdivided into those with an energy reservoir that does not
limit their capability to provide PCR and those with an energy reservoir that limits their capability to
provide PCR. In continental Europe, a PCR unit/group with non-limited energy reservoir shall activate
its PCR for as long as the frequency deviation persists. A PCR unit/group with limited energy
reservoirs shall activate its PCR for as long as the frequency deviation persists, unless its energy
7
reservoir is exhausted in either the positive or negative direction. In addition, a limited energy
unit/group shall be continuously available during normal state, this is defined as a ±50 mHz frequency
band around 50 Hz, and should be able to fully activate PCR continuously for at least 15 minutes
during the alert state. The alert state is reached when the absolute frequency deviation is larger than
100 mHz for at least 5 minutes or larger than 50 mHz for at least 15 minutes. According to the
proposals by the German and Austrian TSOs, [14] and [15], the full activation of PCR by units/groups
with limited energy reservoir needs to be guaranteed for at least 30 minutes in the alert state. These
requirements are equivalent to reserving a 15 minute (Operational Guideline draft) or 30 minute
storage band for the alert state, and therefore limit the permissible energy content during the normal
state, see Figure 7.
Please note that these envisioned 15-30 minute storage bands would have to be reserved in addition to
the BESS energy storage capacity required for either fulfilling typical PCR usage or even for
withstanding the ENTSO-E design worst-case power loss of 3 GW and a frequency deviation of
200 mHz. In the latter case only 7-8 minutes of full PCR activation would be necessary as SCR would
have to start reacting already after 30 seconds and to fully relieve PCR after at most 15 minutes.
Furthermore, it is also unclear how the storage band requirements postulated in [14, 15] come about,
i.e. for what fault case they would actually be necessary, and whether or not these full-activation PCR
requirements would also be demanded of conventional power plants, which is currently not the case.
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
energy content / usable
capacity
energy content / usable
capacity
To put these energy capacity requirements into context, Figure 8 shows a major frequency event
in 2013. During the alert state, the equivalent full activation time was only 8 minutes. It should also be
noted that, currently, most of the major frequency excursions are caused by market-based power
imbalances and not by actual power plant or transmission line outages [16].
permissible
with 30 min
rule
0.5
1
1.5
2
2.5
usable capacity [MWh] / offered
PCR power [MW]
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
permissible
with 15 min
rule
0.5
1
1.5
2
2.5
usable capacity [MWh] / offered
PCR power [MW]
Figure 7 : Permissible energy content in the normal state with the 30 minute rule (left) or 15 minute rule
(right). The 30 minute rule applies in the documents by the German and Austrian TSOs [14] [15], while
the 15 minute rule applies in the Operational Guideline draft.
alert
state
Figure 8 : Frequency profile during a major frequency event on 28/10/2013, courtesy of Swissgrid. The
red area denotes the alert state.
8
Since batteries have a limited storage capacity, such stand-alone-units require a continuous charging
process. Batteries without continuous charging can participate in the PCR only as part of a portfolio
consisting of units/groups with unlimited storage capacity. The current Operational Guidelines draft
does not specify rules for the charging strategies, but some TSOs have formulated rules. The German
TSOs restrict BESS charging while providing PCR to the following strategies [4]:
1. Overfulfillment: PCR providers are allowed to augment the nominal frequency response by up
to 20%. For example, at a frequency of 50.2 Hz, the nominal response is -1 p.u., and
overfulfillment would allow any range between -1 and -1.2 p.u.
2. Deadband: Charging/discharging in the frequency deadband is allowed, as long as no
counterproductive frequency response is generated thereby.
3. Schedule transactions: The charging/discharging offset is to be compensated through schedule
transactions, such as OTC transactions or intra-day transactions.
4. Compensation by other units: The charging/discharging offset can be compensated by other
units, e.g. other power plants.
5. Synchronous time corrections: The deadband could be shifted during these events.
6. Response delay: PCR should be fully activated within 30 seconds. Units that can respond
faster, such as batteries, could choose between an immediate response or a slower response
(i.e. up to 30 seconds for full activation) as part of a charging strategy.
Note that the strategies above do not include the moving average charging strategy by means of
balancing energy as used by the Zurich 1 MW BESS and described in Part II. However, the Swiss
TSO allows this strategy due to the negligible impact it has on SCR, and the significant impact of
ruling out a charging strategy relying on balancing energy on BESS economics.
In the following, we discuss the rules above with regard to effects on BESS sizing requirements. We
focus mainly on the above given charging strategies 3 and 4, since the other strategies cannot be used
as effective charging strategies in their own right since they only bring negligible reductions for the
BESS sizing requirements. The key characteristics of the analyzed charging strategies are listed in
Table 3. All the strategies are based on the moving average concept described in Part II, with a
window length (averaging period) of 15 minutes. Also, for all strategies, the maximum offset power is
set to 0.3 MW. However, different constraints are imposed on the offset signal of the charging
strategies. The base strategy is the Zurich 1 MW BESS strategy described in Part II. The strategy
“compensation by SCR capable unit(s)” assumes that the offset can be compensated by one or several
SCR capable units. Therefore, it is assumed that the delay in compensating the offset is 30 seconds
and that 5 minutes are required until full activation. In the intra-day market strategy, the offset needs
to be compensated by intra-day trades. Therefore, the delay is at least 31 minutes (due to gate closure
30 minutes before delivery) and the offset needs to be kept constant for 15 minutes. Exemplary offset
trajectories for the different strategies are shown in Figure 9. The Zurich BESS strategy and the SCR
unit strategy are very similar, the main difference being the slight delay of 25 seconds between the
two.
Table 3: Charging strategy characteristics.
Minimum delay
Zurich BESS
strategy
5s
Maximum ramp-rate
0.13 MW/min1
Time-step length
1s
Compensation by
SCR capable unit(s)
30 s
0.2 p.u. offset/min
(0.06 MW/min)
1s
Intra-day market
31 min
15 min
1
Implicit maximum ramp rate due to the moving average averaging window of 15 minutes. The maximum
theoretical offset change is 2 MW/15 minutes. However, this is unlikely in practice, where only slower ramp
rates are observed.
9
Figure 9 : Exemplary offset trajectories for the different charging strategies.
The necessary battery capacity requirements for providing 1 MW PCR, assuming a maximum offset
power of 0.3 MW and a round-trip efficiency of 85%, for the different strategies are shown in Table 4.
To obtain these values, two frequency profiles with a sampling time of 1 second are used to test the
feasibility of a given capacity configuration. The first frequency profile is a synthetic “worst-case”
normal state frequency profile (-199 mHz for 5 minutes, -99 mHz for the following 10 minutes, and 49 mHz for the following hour). The second frequency profile is based on the measured frequency in
Switzerland for 1 year, starting on 23 November 2014. The battery sizing complies with both the
synthetic worst-case profile and the measured yearly time series. In practice it is the conservative
worst-case profile which determines the dimensioning, since such frequency excursions, although still
considered “normal”, do not occur in practice. Results in Table 4 show that a charging strategy based
on intra-day trading would require more than double the energy capacity with the other strategies,
mainly because of the long delay of over 30 minutes. The 15 minute and 30 minute full activation
requirements in alert state basically induce an additional need of 500 kWh and 1000 kWh capacity,
respectively, compared with the case where no such rule is applied. This corresponds to a three to sixfold increase in required BESS energy capacity. Please note that the 15 to 30 minute full activation
requirement would be in addition to the energy capacity that is required for the normal PCR activation.
Table 4: Battery capacity requirements for providing 1 MW PCR
for different PCR prequalification requirements and charging strategies.
PCR usage
requirements
normal PCR usage
normal PCR usage +
±15 minutes full
activation requirement
normal PCR +
±30 minutes full
activation requirement
Zurich BESS
strategy
220 kWh
Compensation by
SCR capable units
230 kWh
720 kWh
730 kWh
1140 kWh
1220 kWh
1230 kWh
1640 kWh
Intra-day market
640 kWh
Conclusion
The nowadays widespread liberalized power and ancillary service market frameworks, i.e. the splitting
of electricity production and control reserve power provision, in principle favour battery technologies
as they allow them to play out two of their key capabilities: first, the decoupling of regulation power
provision from actual (net) energy production and, second, their high power ramping capabilities.
Decoupling makes the provision of ancillary services from battery systems more or less independent
of the price levels at the power markets, due to the absence of baseload power production.
As the payments for Ancillary Services are mostly done for the actual regulation power provision and
only to a small extent for regulation energy, (PCR: no energy payment, SCR: small energy payment),
battery systems can have a competitive edge over conventional power plants. In certain market areas,
such as the PJM interconnection in the US North-East, battery systems can earn additional revenues
due to their high power ramping capabilities and response accuracy. This is not yet the case for
European Ancillary Services markets.
10
However, the Ancillary Service market regulations in place, specifically the detailed workings of
BESS prequalification rules, are crucial for the cost-effective integration of BESS systems into
existing Ancillary Services frameworks.
For BESS units, required energy storage capacity is the key determining cost factor. As we have
shown in this paper, depending on the currently discussed PCR prequalification rules for energyconstrained units, these energy capacity requirements can be three to six-fold larger than would
actually be required for proper technical functioning, i.e. PCR provision according to typical usage
patterns and according to PCR activation requirements for the design worst-case fault (3 GW power
loss) as defined by ENTSO-E.
Too conservative prequalification rules for energy-constrained units can thus make the provision of
PCR by technically proper working BESS units economically unviable. The uncontested technical
advantages of BESS units, high power ramping and energy/power decoupling, could thus not be
leveraged for grid operation. These findings need to be considered in the currently on-going
discussions of prequalification rules for energy-constrained power system units in the ENTSO-E CE
grid region. PCR prequalification rules should be designed using the deterministically quantifiable
energy capacity requirements for withstanding clearly defined contingency cases, be it actual power
plant outages or other power imbalances, i.e. caused by forecast errors of RES production or load
demand as well as market-induced schedule mismatches, instead of relying on rough rules of thumb
that may lead to BESS capacity over-sizing.
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Acknowledgement
The authors would like to thank Marc Scherer (Swissgrid) for his inputs on major frequency events.
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